Stable dispersions (nanofluids) are produced using plasma functionalized multi-walled carbon nanotubes (MWCNTs). To our knowledge, this study presents a first quantitative demonstration of nanofluid stability over extended periods of time (currently tested up to 8 months) and after intense heating. No agglomeration is found to occur in the water and glycol-based nanoiluids after heating at 85 and 170 degrees C, respectively. Significant agglomeration does occur for suspensions produced using the non-polar Therminol (R) VP-1 heat transfer fluid. Optical characterization of the nanofiuids demonstrates that the MWCNTs are highly absorbing over the majority of the solar spectrum, allowing for close to 100% solar energy absorption, even at low concentrations and small collection volumes. These absorption properties coupled with the stability of the nanofluids make them ideal candidates as direct solar thermal collectors. (C) 2014 Elsevier Ltd. All rights reserved.